Color cathode ray tube

ABSTRACT

The present invention provides an excellent color cathode ray tube of a black matrix type which can reduce the irregularities of a phosphor screen, can exhibits the uniformity of screen, can exhibit the favorable quality of screen, and can obtain the image display of high quality. The size T of a boundary portion between a large hole and a small hole of each electron beam aperture formed in a shadow mask is made small and the surface roughness of an inner surface is made small so that the Irregularity index which expresses the uniformity of the phosphor screen can be set to a value not greater than 7%.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a color cathode ray tube, andmore particularly to a color cathode ray tube of a black matrix typehaving phosphor pixels and a black matrix film which surrounds thephosphor pixels and is formed of a non-luminescence light-absorbingmaterial layer on an inner surface of a panel.

[0003] 2. Description of the Related Art

[0004] As a video tube of a television receiver set or a monitor tube ofa personal computer or the like, a color cathode ray tube of a blackmatrix type has been popularly adopted. FIG. 4 is a schematiccross-sectional view for explaining an example of the structure of thecolor cathode ray tube of a black matrix type. The color cathode raytube includes an evacuated envelope which consists of a panel portion20, a funnel portion 21 and a neck portion 22. An anode button 23 forintroducing high voltage is embedded in a portion of the funnel portion21.

[0005] Numeral 24 indicates a shadow mask assembled body and the detailof one example of the shadow mask assembled body 24 is shown in FIG. 5Aand FIG. 5B and will be explained later. Numeral 25 indicates anelectron gun which is housed in the inside of the neck portion 22. Highvoltage introduced through the anode button 23 is applied to theelectron gun 25 through an inner conductive film. Numeral 26 indicates adeflection yoke and is exteriorly mounted on a transitional regionbetween the neck portion 22 and the funnel portion 21 of the evacuatedenvelope. By deflecting three modulated electron beams 27 irradiatedfrom the electron gun 25 in the horizontal direction (X direction) andin the vertical direction (Y direction) using the deflection yoke 26,the electron beams 27 are subjected to a two-dimensional scanning on aphosphor screen 28 thus reproducing images. The high voltage introducedthrough the anode button 10 is also applied to a conductive film whichis formed on the phosphor screen 28. In the drawing, numeral 29indicates a magnetic shield.

[0006] The phosphor screen 28 formed on an inner surface of the panelportion 20, one example of the phosphor screen 28 being shown in detailin FIG. 6 as will be explained later, includes tri-color phosphor pixelswhich are formed of phosphors of three colors generally consisting ofred (R), green (G), blue (B) and which are applied in a dot shape or ina stripe shape and a black matrix film which surrounds the phosphorpixels. A color selection electrode assembled body is arranged close tothe phosphor screen 28 and also faces the phosphor screen 28 in anopposed manner. Here, the color selection electrode assembled body isalso referred to as the shadow mask assembled body 24 and a colorselection electrode is explained as the shadow mask.

[0007]FIG. 5A and FIG. 5B show one example of the shadow mask assembledbody 24 shown in FIG. 4. That is, FIG. 5A and FIG. 5B show the shadowmask assembled body 24 to which a shadow mask, a mask frame and springsare fixedly mounted, wherein FIG. 5A is a side view and FIG. 5B is aplan view. Portions identical with the portions which appear in thepreviously mentioned respective drawings are given same numerals. InFIG. 5A and FIG. 5B, the shadow mask assembled body 24 is constituted ofa shadow mask 241, a mask frame 242 and springs 243. The shadow mask 241includes a main surface 241 a which has a large number of electron beampassage apertures 244 a shape of which is shown in FIG. 7 and which willbe explained later as an example and skirt portions which are bent fromthe main surface 241 a at an approximately right angle. The skirtportions are inserted into the inside of the mask frame 242 and theskirt portions and the mask frame 242 are fixed to each other by weldingat portions indicated by a mark x.

[0008] Further, the springs 243 are fixedly secured by welding torespective sides of the mask frame 242 and constitute portions of amechanism which suspends the shadow mask assembled body 24 in the insideof the panel portion 20. Still further, after shaping, the main surface241 a of the shadow mask constitutes a portion which faces the phosphorscreen constituting an inner surface of the panel portion and isconstituted of an apertured region where the large number of electronbeam passage apertures 244 are formed and an outer peripheral portionconstituting an outside non-apertured region which surrounds theperiphery of the apertured region and is indicated by a dotted line. Themain surface 241 a has an approximately rectangular shape and respectiveradii of curvatures along a long axis, a short axis and a diagonal lineare made different from each other. Such a provision is made to satisfyboth of demands for a color cathode ray tube, that is, the flat feelingof the screen and the maintenance of the mechanical strength of theshaped shadow mask.

[0009] The shadow mask 241 mainly uses aluminum-killed steel as theconstitutional material. Here, along with the recent demand for highdefinition to the color cathode ray tube, a shadow mask having a thinfilm thickness has been used. The color cathode ray tube which adoptssuch a thin shadow mask is liable to be subjected to a phenomenon whichis referred to as mask doming in which a portion of the shadow mask isthermally deformed during the operation of the color cathode ray tube sothat electron beam spots are deviated from given positions on thephosphor screen.

[0010] To cope with such a phenomenon, the mechanism which suspends theshadow mask assembled body in the inside of the panel portion isimproved and further Invar material is used as the constitutionalmaterial of the shadow mask in view of the thermal expansion coefficientand the physical hardness. Such a shadow mask is formed into a maskassembled body in such a manner that an original plate in which a largenumber of electron beam passage apertures are formed at given positionsby etching is blanked in a given shape and, thereafter, is formed bypress forming into a shape consisting of a main surface having anapproximately rectangular shape and formed in an approximately sphericalshape and skirt portions which are contiguously connected to a peripheryof the main surface and are bent by approximately 90 degrees withrespect to the main surface, and the shaped shadow mask is fixedlysecured to the mask frame.

[0011]FIG. 6 is a schematic cross-sectional view showing a portion of anessential part of the color cathode ray tube shown in FIG. 4 in anenlarged form. In FIG. 6, the phosphor screen 28 provided to the innersurface of the panel portion 20 includes tri-color phosphor pixelelements 281 which are formed of phosphors of three colors applied in adot shape or in a stripe shape, a black matrix film 282 which surroundsthe phosphor pixel elements 281 and a metal reflection film 283.Further, as previously explained, the shadow mask assembled body 24 isarranged close to the phosphor screen 28 in an opposed manner.

[0012] The tri-color phosphor pixels 281 are formed in a dot shape andare constituted of red (R) phosphor pixels 281R, green (G) phosphorpixels 281G and blue (B) phosphor pixels 281B. The tri-color phosphorpixels 281 are formed through steps in which, as well known, phosphorslurries of respective colors are applied to an inner surface of thepanel portion on which the black matrix film 282 is formed and,thereafter, exposure indicated by an arrow is performed from positionsof three light sources 30G, 30B, 30R indicated by a phantom lineindividually through the electron beam passage apertures 244 formed inthe shadow mask 241.

[0013]FIG. 7 is a schematic cross-sectional view showing an example ofthe electron beam aperture 244 formed in the shadow mask 241. In FIG. 7,the electron beam aperture 244 exhibits a shape which is formed bycombining two holes which differ in size with respect to a boundaryportion, wherein the large hole (first hole portion) 244L faces thephosphor screen 28 side with respect to the boundary portion 244B andthe small hole (second hole portion) 244S faces the electron gun 25 sidewith respect to the boundary portion 244B. The large hole (first holeportion) 244L and the small hole (second hole portion) 244S change theircurving directions at an inflection point P0. That is, the electron beamaperture 244 formed in the shadow mask shown in FIG. 7 is constituted oftwo hole portions, that is, the large hole (first hole portion) 244L andthe small hole (second hole portion) 244S. As mentioned previously, theelectron beam passage apertures 244 are generally formed by etching thethin metal plate and it is more difficult to form holes in Invarmaterial than to form holes in aluminum killed steel.

[0014]FIG. 8 is a geometric optical exposure profile for explaining thephosphor screen forming exposure. In the drawing, parts identical withthose shown in the previously-mentioned respective drawings are givenwith same numerals. In FIG. 8, an exposure amount profile 31 is used ina following method. That is, the exposure amount profile 31 exhibits amaximum area Dmax at the panel portion 20, wherein corresponding to theincrease of distance in the direction away from the panel portion, theexposure intensity is increased and the area is decreased and assumesthe minimum area Dmin at a top surface. By setting the specification toa given position 311, for example, the phosphor pixels having thenecessary area D is formed. The phosphor screen of such a color cathoderay tube of a black matrix type is disclosed in Japanese PatentPublication 218/1971 and the like, for example.

[0015] In the conventional color cathode ray tube of a black matrix typein which the black matrix film surrounds the phosphor pixels having theabove-mentioned constitution, there have been problems that the coarseirregularities of contrast is generated on the phosphor screen so thatthe uniformity of the screen is degraded and the quality of the screenis damaged so that the image display of high quality cannot be obtained.Particularly, with respect to the monitor tube, beside the demand forhigh definition of the screen, a viewer usually reads images at aposition very close to the screen, that is, approximately 50 cm from thescreen, for example, and hence, there arises a problem that thefluctuation of brightness derived from the irregularities is liable togive fatigue to viewer's eyes.

[0016] To explain this problem using drawings, FIG. 9A and FIG. 9B arefront views of the panel portion 20, wherein FIG. 9A is an overall viewand FIG. 9B is a plan view showing a portion A in FIG. 9A in an enlargedmanner. In these drawings, parts which are indicated by same symbolsused in the previously-mentioned respective drawings correspond toidentical parts. In FIG. 9B, among the phosphor pixels 281 whichconstitute the phosphor screen 28, a plurality of phosphor pixels 281B1,281B2, 281B3, 281R3, 281G4 exhibit non-circular shapes and the redphosphor pixels 281R are pixels of a small diameter.

[0017] These irregular phosphor pixels are formed due to the exposureprofiles shown in FIG. 10B and FIG. 10C. That is, while FIG. 10A showsthe exposure profile of the regular phosphor pixel, FIG. 10B shows theexposure profile which is defective at points c1, d1 and hence, as thephosphor pixels on the phosphor screen, the phosphor pixels which haveirregular shapes such as the blue phosphor pixels 281B2, 281B3 and thered phosphor pixels 281R3 which are partially defective are formed.

[0018] Further, the exposure profile shown in FIG. 10C exhibits ahang-out portion as indicated by points c2, d2 and hence, as thephosphor pixels on the phosphor screen, the phosphor pixels such as theblue phosphor pixel 281B1 and the green phosphor pixel 281G4 havingirregular shapes which have portions thereof projected are formed. Sincethese irregular phosphor pixels are interspersed among the regularphosphor pixels, there arises a problem that the coarse irregularitiesof contrast are generated on the screen so that the uniformity of thescreen is damaged and the quality of images is damaged whereby the imagedisplay of high quality cannot be obtained.

[0019] Such irregularities of the phosphor screen is generated byrespective causes which are respectively present with respect to theshadow mask, the panel, the black matrix film, the phosphor pixels andmetal reflective films and the like which are related with the formationof the phosphor screen. These causes are explained by taking the shadowmask as an example. FIG. 11 is a schematic cross-sectional view ofanother example of the electron beam aperture formed in the shadow mask.In FIG. 11, a large number of electron beam passage apertures 244 formedin the shadow mask 241 are formed such that each electron beam aperture244 is constituted of a large hole (first hole portion) 244L formed inthe panel side, a small hole (second hole portion) 244S formed at theelectron gun side and a boundary portion (third hole portion) 244B whichconnects the large hole (first hole portion) 244L and the small hole(second hole portion) 244S.

[0020] A boundary between the first hole portion 244L and the third holeportion 244B is defined as a first inflection point P1 which reaches thethird hole portion 244B from the first hole portion 244L and a boundarybetween the second hole portion 244S and the third hole portion 244B isdefined as a second inflection point P2 which reaches the third holeportion 244B from the second hole portion 244. That is, in other words,the electron beam aperture 244 formed in the shadow mask shown in FIG.11 is constituted of three portions, that is, the first hole portion,the second hole portion and the third hole portion.

[0021] With respect to the shadow mask having these electron beampassage apertures, the third hole portion 244B which constitutes theboundary portion of the electron beam passage apertures 244 formed inthe shadow mask 241 shown in FIG. 11 has a broad width T in thethickness direction compared to the boundary portion 244B of theelectron beam aperture 244 explained in conjunction with FIG. 7 so thatthe irregularities of the phosphor screen is generated due to theirregular reflection of the exposure beams on the boundary portion.Further, the irregularities of the phosphor screen is also generated dueto the surface roughness of the inner surface of the electron beampassage apertures 244. Further, the thickness of the black matrix film,problems on the particle size of the phosphor per se, the composition offilming, the thickness of the metal reflection film and the like alsoaffect the irregularities of the phosphor screen. The previouslymentioned irregularities of the phosphor screen are generated as theresult of the combination of these respective causes so that therearises a problem that the image display of high quality can not beobtained. In view of such a circumstance, a measure which can solve sucha problem is requested. The evaluation of the irregularities of thephosphor screen largely depends on individuals who judge theirregularities of the phosphor screen with their naked eyes so thatthere has been a problem that the accurate judgement can not beattained. A technique which can quantitatively measure theirregularities of phosphor screen is disclosed in Japanese Laid-openPublication 253497/1998.

[0022] That is, FIG. 12 is a view showing the constitution of oneembodiment of an image measuring method and device disclosed in theabove-mentioned publication. In FIG. 12, numeral 32 indicates a colorcathode ray tube which is subjected to an inspection, numeral 33indicates a camera, numeral 34 indicates an image processor, and numeral35 indicates a signal generator. Using these devices, one or pluralityof physical characteristic quantities which respective pixels have suchas an area of the phosphor dot, an amount of light emission from thephosphor dot per unit area, the irregularities of light emissiondistribution in the inside of the phosphor dot and shapes of thephosphor dots which are subjected to an inspection are extracted, thequantitative criterion are calculated, and the quality of images isevaluated.

[0023] According to this publication, the Irregularity index whichexpresses the uniformity of the phosphor screen can be obtained by amethod in which out of the above-mentioned various physicalcharacteristic quantities, one physical characteristic such as the areaof the phosphor dot or a plurality of physical characteristic quantitiessuch as the area of the phosphor dot and the light emission quantity ofthe phosphor dot per unit area are extracted, a numerical value which isobtained by subtracting the minimum value of the extractedcharacteristic quantity from the maximum value of the extractedcharacteristic quantity is divided by the average value of thecharacteristic quantity, and the obtained value is expressed bypercentage so as to quantify the value.

[0024] That is, to explain the above processing by an equation, theequation is expressed as follows.

Irregularity index (%)=[{(maximum value of characteristicquantity)−(minimum value of characteristic quantity)}/(average value ofcharacteristic quantity)]×100

[0025] Although the quantification of the irregularities becomespossible, problems derived from the generation of the irregularitieshave not been solved. Accordingly, it is an object of the presentinvention to provide an excellent color cathode ray tube of a blackmatrix type which can suppress the generation of the irregularities, canexhibit the excellent uniformity of the screen, can exhibit theexcellent quality of screen and can obtain the image display of highquality.

SUMMARY OF THE INVENTION

[0026] To achieve the above-mentioned object, the present invention hasreduced causes or factors which generate irregularities at respectivesteps relevant to the formation of a phosphor screen so as to enhancethe uniformity of the screen. To describe typical constitutions of thepresent invention, they are as follows.

[0027] (1) In a color cathode ray tube having an evacuated envelopewhich is constituted of a panel portion which includes a phosphor screenhaving phosphor pixels and a black matrix film on an inner surfacethereof and a shadow mask being arranged to face the phosphor screen inan opposed manner and having a large number of electron beam passageapertures, a neck portion housing an electron gun and a funnel portionconnecting the panel portion and the neck portion to each other andmounting a deflection yoke on an outer periphery thereof, theIrregularity index which expresses the uniformity of the phosphor screenis not greater than 7%.

[0028] (2) In the above-mentioned constitution (1), the Irregularityindex is not greater than 5.5%.

[0029] (3) In the above-mentioned constitution (1) or (2), the phosphorpixels are formed in a dot shape.

[0030] (4) In any one of the above-mentioned constitutions (1) to (3),each electron beam aperture is formed in the shadow mask such that alarge hole is formed at the phosphor screen side and a small hole isformed at the electron gun side with respect to a boundary portion, andthe thickness of the boundary portion is set to a value not greater than5 μm.

[0031] (5) In any one of above-mentioned constitutions (1) to (4), withrespect to the shadow mask, the surface roughness of the inner surfaceof the electron beam aperture is set to a value not greater than 0.4 μm.

[0032] (6) In any one of above-mentioned constitutions (1) to (5), theIrregularity index which expresses the uniformity of the black matrixfilm is set to a value not greater than 5%.

[0033] (7) In any one of above-mentioned constitutions (1) to (6), theIrregularity index which expresses the uniformity of the shadow mask isset to a value not greater than 3%.

[0034] (8) A color cathode ray tube includes an evacuated envelope whichis constituted of a panel having a phosphor screen which forms a largenumber of sets of three-color phosphor pixel trios on an inner surfacethereof, a neck housing an electron gun which irradiates three electronbeams toward the phosphor screen in the inside thereof and a funnelwhich connects the panel and the neck to each other, and a shadow maskwhich arranges an apertured region thereof which performs colorselection of landing positions of the three electron beams and in whicha large number of electron beam passage apertures are formed close tothe phosphor screen and makes the apertured region face the phosphorscreen in an opposed manner, wherein each electron beam aperture formedin the shadow mask is constituted of a first hole portion opened at thepanel side, a second hole portion opened at the electron gun side and athird hole portion which connects the first hole portion and the secondhole portion to each other, and with respect to a cross-sectionalprofile shape of the electron beam aperture which is obtained by cuttingalong a plane which includes the center of the electron beam apertureand a tube axis, a boundary between the first hole portion and the thirdhole portion is defined as a first inflection point which reaches thethird hole portion from the first hole portion and a boundary betweenthe second hole portion and the third hole portion is defined as asecond inflection point which reaches the third hole portion from thesecond hole portion, and the distance between the first inflection pointand the second inflection point is set to a value not greater than 5 μm.

[0035] (9) In the above-mentioned constitution (8), material whichconstitutes the shadow mask is Invar.

[0036] (10) In the above-mentioned constitution (8), the surfaceroughness in the vicinity of the third hole portion is set to a valuenot greater than 0.4 μm.

[0037] The present invention is not limited to the above-mentionedconstitutions and constitutions of embodiments which will be explainedlater and various modifications are conceivable without departing fromthe technical spirit of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a schematic cross-sectional view for explaining anexample of the structure of one embodiment of a color cathode ray tubeaccording to the present invention.

[0039]FIG. 2 is a graph showing the relationship between the thicknessof a boundary portion of an electron beam aperture of a shadow mask andthe Irregularity index of a phosphor screen or the like.

[0040]FIG. 3 is a view showing the relationship between the surfaceroughness in the vicinity of the boundary portion of the electron beamaperture of the shadow mask and the Irregularity index of the phosphorscreen or the like.

[0041]FIG. 4 is a schematic cross-sectional view for explaining anexample of the structure of a color cathode ray tube of a black matrixtype.

[0042]FIG. 5A is a side view of a shadow mask assembled body shown inFIG. 4 and

[0043]FIG. 5B is a plan view of the shadow mask assembled body shown inFIG. 4.

[0044]FIG. 6 is a schematic cross-sectional view showing a portion of anessential part of the color cathode ray tube shown in FIG. 4 in anenlarged form.

[0045]FIG. 7 is a schematic cross-sectional view of one example of anelectron beam aperture of a shadow mask.

[0046]FIG. 8 is a view showing a geometric-optical exposure profile forexplaining the phosphor screen forming exposure.

[0047]FIG. 9A is a front view of the panel portion of the color cathoderay tube and

[0048]FIG. 9B is an enlarged view of a portion A in FIG. 9A.

[0049]FIG. 10A is a view showing a regular geometric-optical exposureprofile for explaining the phosphor screen forming exposure and

[0050]FIG. 10B ad FIG. 10C are views showing irregular profiles.

[0051]FIG. 11 is a schematic cross-sectional view of another example ofthe electron beam aperture of the shadow mask.

[0052]FIG. 12 is a view showing the constitution of one embodiment of animage quality measuring method and device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] Preferred embodiments of the present invention are explainedhereinafter in detail in conjunction with attached drawings.

[0054]FIG. 1 is a schematic cross-sectional view for explaining anexample of the structure of one embodiment of a color cathode ray tubeaccording to the present invention. The color cathode ray tube shown inFIG. 1 includes an evacuated envelope which is constituted of a panelportion 51 having a phosphor screen 50 which has phosphor pixels and ablack-matrix film formed of a non-luminance light-absorbing materiallayer on an inner surface, a neck portion 52 housing an electron gun 61and a funnel portion 53 which connects the panel portion 51 and the neckportion 52 to each other.

[0055] The phosphor screen 50 formed on the inner surface of the panelportion 51 includes phosphor pixels which are formed by applyingphosphors of three colors generally consisting of red (R), green (G) andblue (B) in a dot shape or a stripe shape respectively, a black matrixfilm which surrounds these phosphor pixels and is made of anon-luminance light-absorbing material layer such as carbon and a metalreflection film which constitutes a metal back layer. With respect tothe phosphor screen 50, the Irregularity index which expresses theuniformity of phosphor screen which will be explained later is set to avalue not greater than 7%. Further, a shadow mask 54 is arranged at aposition close to the phosphor screen 50. The shadow mask 54 isconstituted of Invar material in view of the thermal expansioncoefficient and the physical hardness.

[0056] The shadow mask 54 is of a type which can maintain the shapethereof which is formed by press molding by itself. The shadow mask 54has peripheral sides thereof welded to a mask frame 57 and is supportedon stud pins 60 which are mounted on inner walls of skirt portions ofthe panel portion 51 in an erected manner by way of suspension springs59 in a suspended manner. Further, a magnetic shield 58 is fixed to anelectron gun 61 side of the mask frame 57.

[0057] A deflection yoke 55 is exteriorly mounted on a transitionalregion between the neck portion 52 and the funnel portion 53 of theevacuated envelope. The deflection yoke 55 deflects three modulatedelectron beams B irradiated from the electron gun 61 in the horizontaldirection (X direction) and the vertical (Y direction) so as to performthe two dimensional scanning on the phosphor screen 50 thus reproducingimages. Further, an inner conductive film 62 formed on an inner surfaceof the funnel portion 53 is served for applying high voltage introducedthrough an anode button to electrodes which constitute a main lens ofthe electron gun 61 and the metal deflection film of the phosphor screen50. Here, numeral 65 indicates the whole color cathode ray tube.

[0058]FIG. 2 and FIG. 3 are views showing the relationship between thecharacteristics of the shadow mask and the Irregularity index whichexpresses the uniformity of the phosphor screen, the black matrix filmand the shadow mask. That is, FIG. 2 shows the relationship between thethickness T of the boundary portion 244 b of the electron beam aperture244 of the shadow mask 241 shown in FIG. 11 and the Irregularity index,while FIG. 3 shows the relationship between the surface roughness in thevicinity of the boundary portion 244 b and the Irregularity index. Here,the irregularities of the hole diameter of the electron beam passageapertures 244 formed in the shadow mask used in both of FIG. 2 and FIG.3 are set within 2%.

[0059] First of all, FIG. 2 shows the characteristics of the colorcathode ray tube having the specification in which the arrangementdistance pitch of the neighboring phosphor pixels of same color is setto 0.26 mm, the shadow mask material is formed of Invar material, theplate thickness of the shadow mask is set to 0.13 mm and the holediameter of the electron beam passage apertures is set to 115 to 120 μmand the size of the screen in the diagonal direction is 51 cm. TheIrregularity index shown in FIG. 2 is a value which is calculated usingthe technique disclosed in the previously mentioned Japanese Laid-openPatent Publication 253497/1998. As the physical characteristic quantityin the publication, the light emission quantity of the phosphor dot perunit area is used with respect to the screen (phosphor screen) of themonitor tube and the area of the electron beam aperture of the shadowmask or the area of the black matrix hole is used with respect to othercases.

[0060] In FIG. 2, when the thickness T of the boundary portion 244 bexceeds 6 μm, that is, when the Irregularity index which expressed theuniformity of the phosphor screen exceeds 8%, the gritty irregularitiesof contrast of the phosphor screen becomes apparent so that theuniformity of the screen is deteriorated and the quality of the screenis damaged whereby the image display of high quality is not obtained.Accordingly, it is necessary to set the Irregularity index whichexpresses the uniformity of the phosphor screen to a value not greaterthan 7%. It is desirable to set the irregular index to a value notgreater than 5.5%. In this case, the quality of the image display can befurther enhanced. When the Irregularity index is not greater than 4%,the presence of the irregularity per se can be ignored. To this end, itis preferable to set the thickness T of the boundary portion 244B to avalue not greater than 5 μm.

[0061] Subsequently, FIG. 3 shows the result of the evaluation which isconfirmed using shadow masks having the different surface roughness ofthe electron beam passage apertures under the same specification as FIG.2. When the surface roughness of the electron beam aperture exceeds 0.5μm, that is, when the Irregularity index which expresses the uniformityof the phosphor screen exceeds 8%, the gritty irregularities of contraston the phosphor screen becomes apparent so that the uniformity of thescreen is deteriorated and the quality of the screen is damaged wherebythe image display of high quality can not be obtained.

[0062] Accordingly, by setting the surface roughness to a value notgreater than 0.4 μm, the image display of high quality can be obtained.Here, data on the hole area of the BM (black matrix) film and the dataon the electron beam aperture area of the mask (shadow mask) in FIGS. 2and 3 are respectively collected during the manufacturing steps of thecolor cathode ray tube.

[0063] Further, although the surface of the shadow mask is usuallyblackened, by increasing the thickness of the blackened film by at least10% compared to the blackened film of a standard specification, theIrregularity index can be improved compared to that of the standardspecification even when the surface roughness is equal. That is, withrespect to the shadow masks having the surface roughness of 0.12 μm and0.30 μm, the irregularity indices of black matrix films using theblackened films of the standard specification are respectively 3.2% and4.0%. By increasing the thickness of the blackened films by 10%, theirregularity indices of the black matrix films are respectively improvedby 3.1%, 3.6%. Corresponding to such an improvement, the Irregularityindex of the phosphor screen is also improved.

[0064] Although the relationship between the characteristics of theshadow mask and the Irregularity index has been explained in theabove-mentioned embodiments, as causes or factors which generate theirregularities on the phosphor screen, as mentioned previously, besidesthe material and the process of the shadow mask, the material and theprocess of the panel, the black matrix film, the phosphor pixels, themetal reflection films or the like are considered. Accordingly, byimproving the characteristics and manufacturing controls of theserespective components, the Irregularity index can be enhanced.

[0065] As has been described heretofore, according to the presentinvention, by controlling the Irregularity index by syntheticallyanalyzing the irregularities generated on the phosphor screen andsearching into causes thereof, the generation of the irregularities canbe suppressed, the uniformity of the screen can be enhanced whereby thecolor cathode ray tube of a black matrix type which can obtain the imagedisplay of high quality with the favorable screen quality can berealized.

What we claim is:
 1. A color cathode ray tube comprising: an evacuatedenvelope which is constituted of a panel having a phosphor screen whichforms a large number of sets of three-color phosphor pixel trios on aninner surface thereof, a neck housing an electron gun which irradiatesthree electron beams toward the phosphor screen in the inside thereof,and a funnel which connects the panel and the neck to each other, and ashadow mask which arranges an apertured region thereof which performsthe color selection of landing positions of the three electron beams andin which a large number of electron beam passage apertures are formedclose to the phosphor screen and makes the apertured region face thephosphor screen in an opposed manner, wherein each electron beamaperture formed in the shadow mask is constituted of a first holeportion opened at the panel side, a second hole portion opened at theelectron gun side and a third hole portion which connects the first holeportion and the second hole portion to each other, with respect to across-sectional profile shape of the electron beam aperture which isobtained by cutting along a plane which includes the center of theelectron beam aperture and a tube axis, a boundary between the firsthole portion and the third hole portion is defined as a first inflectionpoint which reaches the third hole portion from the first hole portionand a boundary between the second hole portion and the third holeportion is defined as a second inflection point which reaches the thirdhole portion from the second hole portion, and the distance between thefirst inflection point and the second inflection point is set to a valuenot greater than 5 μm.
 2. A color cathode ray tube according to claim 1,wherein material which constitutes the shadow mask is Invar.
 3. A colorcathode ray tube according to claim 1, wherein the surface roughness inthe vicinity of the third hole portion is set to a value not greaterthan 0.4 μm.
 4. A color cathode ray tube according to claim 1, whereinthe Irregularity index which expresses the uniformity of the phosphorscreen is not greater than 7%.
 5. A color cathode ray tube according toclaim 4, wherein the Irregularity index is not greater than 5.5%.
 6. Acolor cathode ray tube according to claim 4, wherein the phosphor pixelsare formed in a dot shape.
 7. A color cathode ray tube according toclaim 4, wherein each electron beam aperture is formed in the shadowmask such that a large hole is formed at the phosphor screen side and asmall hole is formed at the electron gun side with respect to a boundaryportion, and the thickness of the boundary portion is set to a value notgreater than 5 μm.
 8. A color cathode ray tube according to claim 4,wherein with respect to the shadow mask, the surface roughness of theinner surface of the electron beam aperture is set to a value notgreater than 0.4 μm.
 9. A color cathode ray tube according to claim 4,wherein the Irregularity index which expresses the uniformity of theblack matrix film is set to a value not greater than 5%.
 10. A colorcathode ray tube according to claim 4, wherein the Irregularity indexwhich expresses the uniformity of the shadow mask is set to a value notgreater than 3%.